The novel process and resulting mirror or thin-film optical filter was designed to overcome some of the drawbacks in using Silver as a mirror coating while retaining it's highly reflective characteristics. Silver has excellent reflectance in the visual and infrared wavelengths but has been generally avoided by designers because of the poor environment and durability problems Silver exhibits.
Silver is a material with excellent optical properties. The use of Silver as an optical thin-film material is extensively described for example in the publication “Thin-film Optical Filters”, H. A. Macleod, Adam Hilger Ltd., Second Edition. Unfortunately, Silver has poor environmental compatibility, since it is, on the one hand, relatively soft and consequently can readily be mechanically damaged and, on the other hand, an impairment of optical properties occurs due to corrosion if the silver mirror is exposed without protection against the environment or specific media.
For this reason Silver layers are frequently interleaved in layered systems where the material selected for the remaining layers is determined by the desired optical properties, such as spectral properties, and also by the necessity of increasing the resistance of the Silver layer to environmental influences.
Oxides, Zinc Sulfide, Nitrides, Fluorides, or metals are frequently used in order to protect Silver in films and mirrors. In particular, oxides are used due to their optical properties, their resistance to environmental factors, and also because of their hardness. Applying the oxide layer, however, can cause a degradation of the Silver. Much of the prior art has disclosed attempts to avoid this problem.
For example DR-OS-33 07 661 suggests first covering the silver layer with a further metal layer comprising aluminum, titanium, tantalum, chromium, manganese or zirconium, onto which further metal layers; and lastly an oxide layer is disposed, comprising indium oxide, tin oxide or a mixed oxide thereof DE-OS-35 43 178 suggests a multilayer covering wherein the silver layer, in turn, is covered by a further metal layer comprising tantalum, tungsten, nickel or iron, which further metal layer, in turn, is covered by an oxide layer, wherein SnO, SiO.sub.2, Al.sub.2 O.sub.3, Ta.sub.2 O.sub.5 or ZrO.sub.2 are suggested as the oxide layers. Similarly U.S. Pat. No. 3,682,528 suggests covering the silver layer with a thin nickel layer, if any further layers are to be applied. According to an alternative embodiment, DE 33 27 256 suggests applying at least one hypostoichiometric oxide layer on the silver, comprising, for example, titanium oxide or titanium nitride or a mixture thereof. DE-A-33 29 504 further suggests covering the silver layer with a dielectric layer wherein the material composition in the region of the transition areas, changes gradually and continuously. Titanium oxide is mentioned, for example, as such a dielectric layer.
U.S. Pat. No. 5,510,173 describes substantially transparent copper and silver plus noble metal coatings. These coating's ability to withstand corrosive environments is improved by over-coating the metal layers with a double coating of dielectric. The first coating is made up of a dielectric based on indium and/or zinc, the second coating is made up of a dielectric based on indium and tin.
Lastly, an environmentally stable silver containing mirror having very high reflection values over a large spectral range is disclosed in U.S. Pat. Nos. 6,275,332 and 6,128,126 which comprises a silver containing layer disposed on a substrate, which is covered by a zinc sulfide layer. To keep the sulfur from being set free during the application, or during the vaporization of the zinc sulfide, and attacking the silver, at least one barrier or intermediate layer is placed between the silver containing layer and the zinc sulfide layer.
The high reflective silver mirror or thin-film optical filter of this invention exhibits better durability than the prior art while still maintaining high reflectivity. The prior art's use of Zinc Sulfide is a difficult material to use for coating in a production environment since Sulfur forms sulfuric acid in the presence of humidity and sulfuric acid contamination is very corrosive to Silver.